Dopamine (DA) modulates brain circuitry relevant to cognition, reward, motor control, and arousal. Perturbed DA signaling is believed to contribute to addiction, ADHD, schizophrenia, and Parkinson's disease. The presynaptic DA transporter (DAT) is a major control point for DA signaling and increasing evidence indicates that altered regulation of DAT may contribute to risk for these disorders. We have chosen the nematode Caenorhabditis elegans as a model system to elucidate novel mechanisms regulating DAT and/or DA signaling. We have demonstrated that DA is important for swimming behavior in the worm, as ablation of the C. elegans gene encoding DAT (
dat-1) results in swimming-induced paralysis (Swip). This behavior is attenuated in animals treated with the vesicular monoamine transporter inhibitor, reserpine, and is dependent on signaling through a postsynaptic, D2-like receptor, DOP-3. To determine novel presynaptic regulators of DA signaling, we undertook a chemical mutagenesis screen to identify reserpine-sensitive Swip animals. Two mutants,
vt31 and
vt34, do not possess mutations in
dat-1, fail to complement and map to the same region on LG1. Similar to
dat-1 null animals, these mutants exhibit robust Swip that is suppressed by deletions in
cat-2 or
dop-3. Over-expression of GFP::DAT-1 does not rescue Swip in these mutants, despite normal trafficking of the protein to the synapse. The mutants are partially resistant to the toxin 6-hydroxydopamine, which requires uptake by DAT-1 to produce DA neuron degeneration, consistent with reduced DAT-1 function. Unlike
dat-1 animals,
vt31 and
vt34 have altered male tail morphology, which we found arises from a premature stop codon in the Runx transcription factor homolog RNT-1. Ongoing experiments will determine whether the
rnt-1 mutation underlies the Swip phenotype in these mutants. Elucidation of the molecular lesion associated with
vt31 and
vt34 may yield important and conserved clues to the presynaptic regulation of DA signaling. Supported by NIH awards MH095044 to RDB, T32 MH065215 (S.B.), F31 MH093102 (A.H.), and MH095044 (R.D.B).